The present invention refers to methods and a device for controlling single or polyphase a.c. power controllers by means of phase angle control of semiconductor switches.
For regulation of the power supplied to an electrical load in an a.c. system, a.c. power controllers are used which are controlled by means of phase angle control of semiconductor switches. Particularly during operation of a single phase a.c. motor, the changing operating conditions of the motor--such as, e.g. starting, stopping and operation at varying loads -require a regulation of the power supplied to the motor in order to protect the system, the motor and the driving mechanism from unnecessary strain.
A device for controlling an a.c. power controller for a single phase a.c. motor is known from British Pat. No. 2,084,359, in which, e.g., an unfavorable power factor that arose through an underloading of the motor is to be improved. For this purpose, controllable semiconductor switches, especially a Triac [bidirectional triode thyristor] or an antiparallel thyristor circuit, are assigned to the motor for each supply phase. By controlling the phase angle, these semiconductor switches make possible a power input that is dependent on the actual operating conditions of the motor. An improvement of the power factor is achieved in this known device in that the phase difference between current and voltage is detected for each phase in a control device and is reduced by a corresponding increase of the firing angle, i.e. the period that elapses between the zero crossing of the current wave and the firing point for the respective phase. In the known device, the time point of the current zero is used as a time reference for the determination of the firing point, which time point is detected by measuring the voltage applied across the Triac. This voltage is supplied to a comparator, whose output states correspond to the conduction states of this Triac. The current zero, then, corresponds to an edge of the output signal of the comparator, from which edge a strobe pulse is generated with the aid of a monoflop for a ramp voltage that is synchronized with the zero crossing of the power supply voltage. The sampled value of the ramp voltage is subtracted from a reference voltage that is externally inputted over a potentiometer and is transmitted to the inverting input of a differential amplifier, whose output voltage, together with the ramp voltage, is supplied to an additional comparator. This comparator generates a primary firing signal by means of a downstream triggering pulse generator whenever the ramp voltage exceeds the output voltage of the differential amplifier. By means of this circuit arrangement, the phase shift between motor current and motor voltage and thus the power factor are thereby stabilized at a value which is given by the reference voltage adjusted at the potentiometer.
In the case of three-phase a.c. motors, which are operated without neutral wires, e.g. in open star- or delta connections, one must be careful during phase angle control that at least two phases are simultaneously conductive at all times. In the case of firing angles that exceed 60.degree., this is the case only when a second phase is fired in addition to the phase initiating the firing. This second phase is defined by the rotational direction of the a.c. voltage and the phase initiating the firing.
In the known device, this is realized by a logic circuit consisting of six AND-gates, of which two respectively, are assigned to the control device of a respective phase. By means of a rotational direction detector, a binary signal is applied according to rotational direction of the a.c. voltage on one of two output lines of the rotational direction detector. Together, respectively, with the output line for the primary firing signal of the phase being released, these output lines are placed at various AND-gates, whose outputs supply, respectively, the gate of one of the two other Triacs with secondary firing signals. The temporal length of the firing signals applied to the gates of the Triacs or thyristors must be adequate in order to guarantee a reliable firing of the respective circuit. This is accomplished in that starting with empirical values, a fixed temporal length is selected that is long enough to bring about a firing under all operating conditions. However, in order to prevent an overload of required ignition amplifiers--especially ignition transformers--too long a duration of the firing signal is undesirable. In practice, this leads to compromises which are not always satisfactory in determining the temporal lengths of the firing signals.